Konstantina Lambrinou

The solubility of zirconium (Zr) in the Nb4AlC3 host lattice was investigated by combining the experimental synthesis of (Nbx, Zr1-x)4AlC3 solid solutions with density functional theory calculations. High-purity solid solutions were prepared by reactive hot pressing of NbH0.89, ZrH2, Al and C starting powder mixtures. The crystal structure of the produced solid solutions was determined using X-ray and neutron diffraction. The limited Zr solubility (maximum 18.5% of the Nb-content in the host lattice) in Nb4AlC3 observed experimentally is consistent with the calculated minimum in the energy of mixing. The lattice parameters and microstructure were evaluated over the entire solubility range, while the chemical composition of (Nb0.85, Zr0.15)4AlC3 was mapped using atom probe tomography. The hardness, Young’s modulus and fracture toughness at room temperature, as well as the high-temperature flexural strength and E-modulus of (Nb0.85, Zr0.15)4AlC3 were investigated and compared to those of pure Nb4AlC3. Quite remarkably, an appreciable increase in fracture toughness was observed from 6.6 +/- 0.1 MPa.m1/2 for pure Nb4AlC3 to 10.1 +/- 0.3 MPa.m1/2 for the (Nb0.85, Zr0.15)4AlC3 solid solution.

This work explored a new method to textureMn+1AXn phases by deforming discs of dense ceramics Maxthal 211 (nominally Ti2AlC) and Maxthal 312 (nominally Ti3SiC2) in a spark plasma facility. The disc diameter increased by 40% without crack formation. After deformation, the MAX grains exhibited clear preferential crystallographic orientation, whereby the c-axis was aligned to the compression direction. The fracture toughness increased both parallel and perpendicular to the textured top surface compared with the as-sintered materials. The onset of plastic deformation was observed near the brittle-to-plastic transformation temperature, a fact that could be associated with the change in elastic response.

Chemically complex MAX phase-based ceramics in the (Ti,Zr,Hf,V,Nb)-(Al,Sn)-C system were synthesised by reactive hot pressing for 30 min at 1250–1450°C under a load of 30 MPa for the first time in this work. The dense bulk ceramics contained chemically complex double solid solution MAX phases, each comprising five M-elements and two A-elements. The predominant 211 (Ti0.23,Zr0.18,Hf0.20,V0.11,Nb0.28)2(Al0.42,Sn0.58)C MAX phase characterised all ceramics, while the 312 (Ti0.23,Zr0.31,Hf0.31,V0.08,Nb0.08)3(Al0.36,Sn0.64)C2 and 211 (Ti0.26,Zr0.07,Hf0.07,V0.47,Nb0.13)2(Al0.66,Sn0.34)C MAX phases were only present in the ceramics sintered at 1350–1450°C. A limited amount (4–5 vol%) of parasitic phases (mainly, binary intermetallics) was found in the pseudo-binary carbide-free ceramics sintered at 1350–1450°C.

In this work, several ceramic materials were exposed together with two reference structural materials (i.e., 316L stainless steel and Inconel 600) to a molten solar salt (40 wt% KNO3 and 60 wt% NaNO3) for 1000 h at 600°C to investigate their compatibility with the molten salt medium, thus assessing their potential use in concentrated solar power (CSP) applications. The exposed ceramics included different SiC grades (solid state-sintered, liquid phase-sintered, and silicon-infiltrated) and MAX phase-based materials (Maxthal® 211 & 312 (nominally, Ti2AlC & Ti3SiC2, respectively), Cr2AlC, Nb4AlC3, (Nb,Zr)4AlC3, and a cermet comprising 40 vol% Fe and 60 vol% (Nb,Zr)4AlC3). All SiC grades were chemically stable in the molten salt, whereas all Nb-containing MAX phase ceramics were severely oxidized. Comparing the two Maxthal® grades showed that the 312 was chemically more stable than the 211, and both grades formed a Na-based oxide scale. Interestingly, Cr2AlC showed practically no interaction with the molten salt during the performed exposure, forming a stable, sub-micrometre-thick Cr7C3 scale. Hence, it may be considered as promising structural/coating material candidate for the targeted CSP application.

This work investigates the compatibility of Zr2AlC MAX phase-based ceramics with liquid LBE, and proposes a mechanism to explain the observed local Zr2AlC/LBE interaction. The ceramics were exposed to oxygen-poor (CO ≤ 2.2×10−10 mass%), static liquid LBE at 500°C for 1000 h. A new Zr2(Al,Bi,Pb)C MAX phase solid solution formed in-situ in the LBE-affected Zr2AlC grains. Out-of-plane ordering was favorable in the new solid solution, whereby A-layers with high and low-Bi/Pb contents alternated in the crystal structure, in agreement with first-principles calculations. Bulk Zr2(Al,Bi,Pb)C was synthesized by reactive hot pressing to study the crystal structure of the solid solution by neutron diffraction.

Several steels were exposed to either static or flowing liquid lead-bismuth eutectic under various exposure conditions. Steels T91, EP-823, S2439 and S2440 were exposed to oxygen-rich static LBE at 490C for about 5016 h. The experiments in flowing LBE were carried out in the CORRIDA loop at 550C and around 10-6 mass% dissolved oxygen. The steels tested in the CORRIDA loop included the reduced activation steel EUROFER 97 and two heats of an oxide dispersion strengthened steel produced by mixing EUROFER 97 and yttria powders. The exposure time varied between 1007 and 7511 h for the EUROFER 97 steel and between 5012 and 20039 h for the two ODS heats. The exposed steels were characterized by means of scanning electron microscopy and energy-dispersive X-ray spectrometry.

This work reports on sputter depositions carried out from a compound (Ti,Zr)2AlC target on Al2O3(0 0 0 1) substrates at temperatures ranging between 500 and 900°C. Short deposition times yielded 30–40 nm-thick Al-containing (Ti,Zr)C films, whereas longer depositions yielded thicker films up to 90 nm which contained (Ti,Zr)C and intermetallics. At 900°C, the longer depositions led to films that also consisted of solid solution MAX phases. Detailed transmission electron microscopy showed that both (Ti,Zr)2AlC and (Ti,Zr)3AlC2 solid solution MAX phases were formed. Moreover, this work discusses the growth mechanism of the thicker films, which started with the formation of the mixed (Ti,Zr)C carbide, followed by the nucleation and growth of aluminides, eventually leading to solid state diffusion of Al within the carbide, at the highest temperature (900°C) to form the MAX phases.

Materials subjected to irradiation damage often undergo local microstructural changes that can affect their expected performance. To investigate such changes, this work proposes a novel approach to detect strain localisation caused by irradiation-induced damage in nuclear materials on the microstructural level, considering a statistically relevant number of grains. This approach determines local strains using high- resolution digital image correlation (HRDIC) and compares them with the underlying material microstructure. Sets of images captured before and after irradiation are compared to generate full-field displacement maps that can then be differentiated to yield high-resolution strain maps. These strain maps can subsequently be used to understand the effects of irradiation-induced dimensional change and cracking on the microscale. Here, the methodology and challenges involved in combining scanning electron microscopy (SEM) with HRDIC to generate strain maps associated with radiation-induced damage are presented. Furthermore, this work demonstrates the capabilities of this methodology by analysing three different materials subjected to proton irradiation: a zircaloy-4 (Zry-4) metal irradiated to 1 & 2 dpa, and two ceramics based on MAX phase compounds, i.e., the Nb4AlC3 ternary compound and a novel (Ta,Ti)3AlC2 solid solution, both irradiated to -0.1 dpa. These results demonstrated that all materials show measurable expansion, and the very high strains seen in the MAX phase ceramics can be easily attributed to their microstructure. Grain-to-grain variability was observed in Zry-4 with a macroscopic expansion along the rolling direction that increased with irradiation damage dose, the Nb4AlC3 ceramic showed significant expansion within individual grains, leading to intergranular cracking, while the less phase-pure (Ta,Ti)3AlC2 ceramic exhibited very high strains at phase boundaries, with limited expansion in the binary carbide phases. This ability to measure irradiation-induced dimensional changes at the microstructural scale is important for designing microstructures that are structurally resilient during irradiation.

The effect of severe plastic deformation on the deuterium retention in tungsten exposed to high-flux low-energy plasma (flux ~1024 m_2 s_1, energy ~50 eV and fluence up to 51025 D/m2) was studied experimentally in a wide temperature range (460-1000 K) relevant for application in ITER. The desorption spectra in both reference and plastically-deformed samples were deconvoluted into three contributions associated with the detrapping from dislocations, deuterium-vacancy clusters and pores. As the exposure temperature increases, the positions of the release peaks in the plastically-deformed material remain in the same temperature range but the peak amplitudes are altered as compared to the reference material. The desorption peak attributed to the release from pores (i.e. cavities and bubbles) was suppressed in the plastically deformed samples for the low-temperature exposures, but became dominant for exposures above 700 K. The observed strong modulation of the deuterium storage in “shallow” and “deep” traps, as well as the reduction of the integral retention above 700 K, suggest that the dislocation network changes its role from “trapping sites” to “diffusion channels” above a certain temperature. The major experimental observations of the present work are in line with recent computational assessment based on atomistic and mean field theory calculations available in literature.

This work addresses the dissolution corrosion behaviour of 316L austenitic stainless steels. For this purpose, solution-annealed and cold-deformed 316L steels were simultaneously exposed to oxygen-poor (< 10-8 mass%) static liquid lead-bismuth eutectic (LBE) for 253-3282 h at 500C. Corrosion was consistently more severe for the cold-drawn steels than the solution-annealed steel, indicating the importance of the steel thermomechanical state. The thickness of the dissolution-affected zone was non-uniform, and sites of locally-enhanced dissolution were occasionally observed. The progress of LBE dissolution attack was promoted by the interplay of certain steel microstructural features (grain boundaries, deformation twin laths, precipitates) with the dissolution corrosion process. The identified dissolution mechanisms were selective leaching leading to steel ferritization, and non-selective leaching; the latter was mainly observed in the solution-annealed steel. The maximum corrosion rate decreased with exposure time and was found to be inversely proportional to the depth of dissolution attack.

This work addresses the early stages (≤1000 h) of the dissolution corrosion behavior of 316L and DIN 1.4970 austenitic stainless steels in contact with oxygen-poor (CO < 10-8 mass%), static liquid lead bismuth eutectic (LBE) at 500°C for 600–1000 h. The objective of this study was to determine the relative early-stage resistance of the uncoated steels to dissolution corrosion and to assess the protectiveness of select candidate coatings (Cr2AlC, Al2O3, V2AlxCy). The simultaneous exposure of steels with intended differences in microstructure and thermomechanical state showed the effects of steel grain size, density of annealing/deformation twins, and secondary precipitates on the steel dissolution corrosion behavior. The findings of this study provide recommendations on steel manufacturing with the aim of using the steels to construct Gen-IV lead-cooled fast reactors.

This work addresses the effect of deformation twinning on the dissolution corrosion behaviour of 316L austenitic stainless steels in contact with static liquid lead-bismuth eutectic (LBE). For this purpose, plastically deformed 316L steel specimens with distinctly different deformation twin densities were simultaneously exposed to oxygen-poor (<10_13 mass%) static liquid LBE for 1000 h at 500°C. The variation in deformation twin density was achieved by loading in uniaxial tension to similar degrees of plastic deformation (8-10%) specimens made of the same 316L steel heat. Tensile loading was carried out at -150, 25 and 150°C so as to affect the twin density, which increased as the temperature of plastic deformation decreased. Dissolution corrosion was the only liquid metal corrosion mechanism observed in the LBE-exposed steel specimens. The thickness of the dissolution-affected zone increased with the deformation twin density, which was highest in the 316L steel specimen deformed at -150°C and lowest in the one deformed at 150°C. As deformation twin boundaries accelerated the LBE ingress into the steel bulk, their local orientation with respect to the steel specimen surface affected the thickness of the dissolution-affected zone.

This work focuses on the effect of dissolved oxygen concentration in liquid lead-bismuth eutectic (LBE) on the onset of dissolution corrosion in a solution-annealed 316L austenitic stainless steel. Specimens made of the same 316L stainless steel heat were exposed for 1000 h at 450°C to static liquid LBE with controlled concentrations of dissolved oxygen, i.e., 10_5, 10_6, and 10_7 mass%. The corroded 316L steel specimens were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). A complete absence of dissolution corrosion was observed in the steel specimens exposed to liquid LBE with 10_5 and 10_6 mass% oxygen. In the same specimens, isolated “islands” of FeCr-containing oxides were also detected, indicating the localized onset of oxidation corrosion under these exposure conditions. On the other hand, dissolution corrosion with a maximum depth of 59 μm was detected in the steel specimen exposed to liquid LBE with 10_7 mass% oxygen. This suggests that the threshold oxygen concentration associated with the onset of dissolution corrosion in this 316L steel heat lies between 10_6 and 10_7 mass% oxygen for the specific exposure conditions (i.e., 1000 h, 450°C, static liquid LBE).

The effect of severe plastic deformation on the deuterium retention in tungsten exposed to high-flux low-energy plasma (flux 1024 D/m2/s, energy 50 eV, and fluence up to 31026 D/m2) at the plasma generator Pilot-PSI was studied by thermal desorption spectroscopy and scanning electron microscopy. The desorption spectra in both reference and plastically deformed samples were deconvolved into three contributions attributed to the detrapping from dislocations, deuterium-vacancy clusters, and pores, respectively. The plastically induced deformation, resulting in high dislocation density, does not change the positions of the three peaks, but alters their amplitudes as compared to the reference material. The appearance of blisters detected by scanning electron microscopy and the desorption peak attributed to the release from pores (i.e., deuterium bubbles) were suppressed in the plastically deformed samples but only up to a certain fluence. Beyond 51025 D/m2, the release from the bubbles in the deformed material is essentially higher than in the reference material. Based on the presented results, we suggest that a dense dislocation network increases the incubation dose needed for the appearance of blisters, associated with deuterium bubbles, by offering numerous nucleation sites for deuterium clusters eventually transforming into deuterium-vacancy clusters by punching out jogs on dislocation lines.

Liquid lead (Pb)- and lead–bismuth eutectic (LBE)-cooled fast neutron reactors (Gen-IV LFRs) are one of the most technologically mature fission reactor technologies, due to their inherent safety, high power density, and ability to burn nuclear waste. Accelerator-driven systems (ADS), in particular, promise to address the issues of long-lived radiotoxic nuclear waste, emerging uranium ore shortages, and the ever-increasing demand for energy. However, the conditional compatibility of conventional structural materials, such as steels, with liquid Pb and liquid LBE is still an important concern for the deployment of these advanced nuclear reactor systems, making the environmental degradation of candidate structural and fuel cladding steels the main impediment to the construction of Gen-IV LFRs, including ADS. This article presents a comprehensive review of the current understanding of environmental degradation of materials in contact with liquid Pb and liquid LBE, with a focus on the underlying mechanisms and the factors affecting liquid metal corrosion (LMC) and liquid metal embrittlement (LME), which are the two most important materials degradation effects. Moreover, this article addresses the most promising LMC and LME mitigation approaches, which aim to suppress their adverse influence on materials performance. An outlook of the needed future work in this field is also provided.

Zr2AlC MAX phase-based ceramic material with 33 wt% ZrC has been irradiated with 22 MeV Au7+ ions between room temperature and 600°C, achieving a maximum nominal midrange dose of 3.5 displacements per atom. The response of the material to irradiation has been studied using scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Under room temperature irradiation, the ions caused a partial amorphisation of the MAX phase. At high temperatures, irradiated Zr2AlC remained crystalline, but developed an increased density of dislocations and stacking faults in the (0001) basal planes. The irradiated material also exhibited a temperature-dependent microcracking phenomenon similar to that previously reported in other MAX phase materials.

In the present work, we apply a high-throughput density functional theory (DFT) screening of interesting M2AX phase compounds for nuclear applications by assessing their mechanical stability. Evaluation of mechanical stability allows to assess thermodynamically unstable phases and does not require the assessment of competing MX and intermetallic phases. We consider all possible combinations with M = {Ti, Cr, Zr, Nb}, A = {Al, Si, Sn, Pb, Bi} and X = {C}, including “out-of-plane” ordering that is so far unobserved in M2AX phases. For all fifty possible combinations, we determine the elastic constants and verify their mechanical stability. In addition, for each combination, the free surface energy is computed and the fracture toughness, KIC, is determined. The results are discussed in terms of combinations with high mechanical stability and high KIC. Apart from suggestions of interesting new combinations, the results also form the basis for any plasticity or fracture mechanics model for these MAX phases.

Thin foils of the double solid solution (Zr0.5,Ti0.5)2(Al0.5,Sn0.5)C MAX phase were in situ irradiated in a transmission electron microscope (TEM) up to a fluence of 1.3×1017 ions cm-2 (∼7.5 dpa), using 6 keV He+ ions. Irradiations were performed in the 350–800°C temperature range. In situ and post-irradiation examination (PIE) by TEM was used to study the evolution of irradiation-induced defects as function of dose and temperature. Spherical He bubbles and string-like arrangements thereof, He platelets, and dislocation loops were observed. Dislocation loop segments were found to lie in non-basal-planes. At irradiation temperatures ≥450°C, grain boundary tearing was observed locally due to He bubble segregation. However, the tears did not result in transgranular crack propagation. The intensity of specific spots in the selected area electron diffraction patterns weakened upon irradiation at 450 and 500°C, indicating an increased crystal symmetry. Above 700°C this was not observed, indicating damage recovery at the high end of the investigated temperature range. High-resolution scanning TEM imaging performed during the PIE of foils previously irradiated at 700°C showed that the chemical ordering and nanolamination of the MAX phase were preserved after 7.5 dpa He+ irradiation. The size distributions of the He platelets and spherical bubbles were evaluated as function of temperature and dose.

This study addresses the effect of plastic deformation on the dissolution corrosion behavior of a Type 316L austenitic stainless steel. Dissolution corrosion was promoted by low oxygen conditions in liquid lead-bismuth eutectic (LBE). Specimens with controlled degree of plastic deformation (20%, 40%, and 60%) and a non-deformed, solution-annealed specimen were simultaneously exposed for 1,000 h at 500°C to static LBE with low oxygen concentration ([O] < 10−11 mass%). The corroded specimens were analyzed by various material characterization techniques. All exposed specimens exhibited dissolution corrosion. The non-deformed steel showed the least dissolution attack (maximum depth: 36 μm), while the severity of attack increased with the degree of steel deformation (maximum depth in the 60% steel: 96 μm). It was, thus, concluded that increasing the amount of plastic deformation in a Type 316L stainless steel results in higher dissolution corrosion damages for steels exposed to low oxygen LBE conditions. Additionally, it was observed that the presence of chemical bands and δ-ferrite inclusions in a Type 316L steel affected its dissolution corrosion behavior.

This manuscript presents important material challenges regarding innovative Gen-II/III nuclear systems and research reactors. The challenges are discussed alongside the key achievements so far realised within the framework of 4 EU-funded projects: H2020 IL TROVATORE, FP7 MULTIMETAL, FP7 MATTER and FP7 SCWR-FQT. All the four Projects deal with innovative research on materials to enhance the safety of nuclear reactors. IL TROVATORE proposes new materials for fuel cladding of PWR reactors and tests in order to really find out an “Accident Tolerant Fuel” (ATF). MULTIMETAL focused on optimization of dissimilar welds fabrication having considered the field performances and dedicated experiments. MATTER carried on methodological and experimental studies on the use of grade 91 steel in the harsh environment of liquid metal cooled EU fast reactors. SCWR-FQT focused on fuel qualification of Supercritical Water Reactor including the selection of the better material to resist the associated high thermal flux.

Select MAX phase-based ceramics were screened with respect to their potential susceptibility to environmentally-assisted degradation in an oxygen-poor liquid lead-bismuth eutectic (LBE) environment, both under static and fast-flowing exposure conditions. The majority of the MAX phases exposed to oxygen-poor (CO ≤ 2.210_10 mass%) static liquid LBE for at least 1000 h at 500°C showed exceptional chemical compatibility with the heavy liquid metal, i.e., no evidence of LBE dissolution attack was observed, despite the absence of a continuous oxide scale on the surface of the exposed MAX phase ceramics. The local LBE interaction observed only with the Zr-rich MAX phases consisted in the partial substitution of Al by Pb/Bi in the MAX phase crystal structure and the in-situ formation of Pb/Bi-containing solid solutions. Moreover, the interaction of Zr-based MAX phases with static liquid LBE was accompanied by the dissolution of parasitic intermetallic phases, which facilitated the further LBE ingress into the ceramic bulk. The erosion resistance of select MAX phase ceramics was also assessed in oxygen-poor (CO  510_9 mass%) fast-flowing (v  8 m/s) liquid LBE for 1000 h at 500°C. Despite the moderate LBE oxygen concentration, oxidation was the predominant corrosion mechanism, while no erosion damages were observed in the exposed MAX phase ceramics. The resistance of the MAX phase ceramics to both dissolution corrosion and erosion in contact with oxygen-poor static and fast-flowing liquid LBE, respectively, was compared to that of the 316L reference structural stainless steel.

Although liquid metals are effective fluids for heat transfer, pumping them at high temperatures is limited by their corrosiveness to solid metals. A clever pump design addresses this challenge using only ceramics.

This chapter discusses how MAX phases combine unique mechanical and thermal properties with good corrosion resistance and promising radiation tolerance. Their unique combination of properties makes some of the MAX phases attractive for harsh service conditions such as those established in the core of a nuclear reactor system. More specifically, the cladding tubes containing the fuel pellets are subjected to elevated temperatures and high neutron irradiation doses while being exposed to a usually corrosive primary coolant. The chapter also examines the influence of the chemical starting composition on the final phase assembly of the hot pressed ceramics, revealing that selecting the appropriate Al and C contents in the starting powder mixture is crucial. Phase-pure ceramics were not obtained under the investigated conditions. One of the major challenges in the development of lead-cooled fast reactors (Gen-IV LFRs) is the inherent corrosiveness of the heavy liquid metal coolant for most structural and cladding steels. Undesirable liquid metal corrosion effects degrade all stainless steels exposed to liquid lead (Pb) and lead-bismuth eutectic (LBE) due to the dissolution of steel alloying elements (Ni, Mn, Cr, Fe) in the liquid metal, which becomes severe at high temperatures and low oxygen contents in the liquid metal coolant.

There is a clear need for high-strength (≥300 MPa), thermally stable, conductive materials that are also thermal shock resistant. Some MAX phases – ternary nano-laminated carbides and nitrides – are reported to fulfil all these requirements and can be considered as potential structural materials for high-temperature applications. In this work, a set of quaternary (M,M′)AX phase materials based on the Nb-Al-C system were synthesized by reactive hot pressing, starting from M-hydride powders. The possibility to substitute Nb with at least 10 at% of other M elements (M′= Ti, Zr, Hf and Ta) in the crystal lattice was investigated. The crystal structure of the produced solid solutions was studied by X-ray diffraction and the lattice parameters were calculated by Rietveld refinement. The material behaviour in an inert atmosphere was tested by measuring the elastic properties – Young's Modulus and internal friction – as a function of temperature up to 1500°C, and the effect of the substitution on the room temperature flexural strength was assessed.

: Kinking is a deformation mechanism ubiquitous to layered systems, ranging from the nanometer scale in layered crystalline solids, to the kilometer scale in geological formations. Herein, we demonstrate its origins in the former through multiscale experiments and atomistic simulations. When compressively loaded parallel to their basal planes, layered crystalline solids first buckle elastically, then nucleate atomic-scale, highly stressed ripplocation boundaries – a process driven by redistributing strain from energetically expensive in-plane bonds to cheaper out-of-plane bonds. The consequences are far reaching as the unique mechanical properties of layered crystalline solids are highly dependent upon their ability to deform by kinking. Moreover, the compressive strength of numerous natural and engineered layered systems depends upon the ease of kinking or lack thereof.

The orientation relationship of an austenite-to-ferrite phase transformation in 316L stainless steels induced by the loss of austenite stabilizers resulting from the steel dissolution corrosion in liquid Pb-Bi eutectic was studied by means of electron backscatter diffraction. The misorientations at the austenite/ferrite interface were compared to the prevailing orientation relationship models in steels. The Pitsch orientation relationship model was found to be predominant, which is unusual for austenite-to-ferrite bulk transformations in steels. The nature of this particular transformation, which involves loss of steel alloying elements and the presence of an interfacial liquid metal layer, is discussed to explain this finding.

Mould casting and sacrificial templating techniques, common in bioceramic technology, were employed to process porous TaCx ultra-high temperature ceramics intended as novel target materials for isotope separation on-line (ISOL) facilities, aiming primarily at the production of medical radioisotopes. A feedstock of Ta4AlC3 MAX phase powder, polyamide spheres and wax was used to obtain different porous TaCx grades with bimodal pore size distributions. The ‘green’ bodies underwent de-binding and vacuum annealing to decompose the MAX phase, whereas a reference material was also produced from commercial TaC powders. The thermal stability of the porous TaCx ceramics was assessed at ISOL-relevant conditions by heating in high vacuum up to 2200°C. The MAX phase-derived TaCx porous ceramics evolved from biphasic TaCx/α-Ta2C to single-phase TaCx at higher temperatures, due to carbon incorporation. The porous TaCx microstructure was stable at 2200°C with a specific surface area stabilizing at ~0.25 m2/g and thermal conductivity of 1-4 W/m K.

Fine-grained Ti2SnC was successfully synthesized and densified by spark plasma sintering (SPS) of pure Ti, Sn and C powders at 1200–1400°C. Ti2SnC was the predominant phase at 1325°C, with minor amounts of remaining Sn and TiCx. At lower processing temperatures, Ti–Sn intermetallics and TiCx were present, whereas the decomposition of the MAX phase into Sn and TiCx was observed at higher temperatures. The shear modulus, Young’s modulus and Poisson’s ratio were measured to be 83.9 GPa, 207.4 GPa and 0.24, respectively. The evolution of the Young’s modulus as a function of temperature was measured up to 800°C.

This work studied the effect of adding 10 at% Fe, Co or Ni to M-Sn-C mixtures with M = Ti, Zr or Hf on MAX phases synthesis by reactive spark plasma sintering. Adding Fe, Co or Ni assisted the formation of 312 MAX phases, i.e., Ti3SnC2, Zr3SnC2 and Hf3SnC2, while their 211 counterparts Ti2SnC, Zr2SnC and Hf2SnC formed in the undoped M-Sn-C mixtures. The lattice parameters of the newly synthesized Zr3SnC2 and Hf3SnC2 MAX phases were determined by X-ray diffraction. Binary MC carbides were present in all ceramics, whereas the formation of intermetallics was largely determined by the selected additive. The effect of adding Fe, Co or Ni on the MAX phase crystal structure and the microstructure of the produced ceramics was investigated in greater detail for the case of M = Zr. A mechanism is herein proposed for the formation of M3SnC2 MAX phases.

Taking the example of tungsten, we demonstrate that high-flux plasma exposure of recrystallized and plastically deformed samples leads to principal differences in the gas trapping and associated surface modification. Surface of the exposed pre-deformed samples exhibits ruptured μm-sized blisters, a signature of bubbles nucleated close to the surface on the plastically induced dislocation network. Contrary to the recrystallized samples, no stage attributable to gas bubbles appeared in the desorption spectrum of the deformed samples demonstrating the strong impact of dislocations on hydrogen retention.

Quasi phase-pure (>98 wt%) MAX phase solid solution ceramics with the (Zr,Ti)2(Al0.5,Sn0.5)C stoichiometry and variable Zr/Ti ratios were synthesized by both reactive hot pressing and pressureless sintering of ZrH2, TiH2, Al, Sn, and C powder mixtures. The influence of the different processing parameters, such as applied pressure and sintering atmosphere, on phase purity and microstructure of the produced ceramics was investigated. The addition of Sn to the (Zr,Ti)2AlC system was the key to achieve phase purity. Its effect on the crystal structure of a 211-type MAX phase was assessed by calculating the distortions of the octahedral M6C and trigonal M6A prisms due to steric effects. The M6A prismatic distortion values were found to be smaller in Sn-containing double solid solutions than in the (Zr,Ti)2AlC MAX phases. The coefficients of thermal expansion along the ⟨a⟩ and ⟨c⟩ directions were measured by means of Rietveld refinement of high-temperature synchrotron X-ray diffraction data of (Zr1−x,Tix)2(Al0.5,Sn0.5)C MAX phase solid solutions with x = 0, 0.3, 0.7, and 1. The thermal expansion coefficient data of the Ti2(Al0.5,Sn0.5)C solid solution were compared with those of the Ti2AlC and Ti2SnC ternary compounds. The thermal expansion anisotropy increased in the (Zr,Ti)2(Al0.5,Sn0.5)C double solid solution MAX phases as compared to the Zr2(Al0.5,Sn0.5)C and Ti2(Al0.5,Sn0.5)C end-members.

MAX phase ceramics are typically prepared by the reactive sintering of elemental powders that are often coarse, expensive, and prone to oxidation. The temperature-driven dehydrogenation of metal hydride powders offers an alternative synthesis approach, as the hydrides decompose into phase-pure, dimensionally fine elemental powder particles. The increased reactivity of these in situ formed, fine powder particles drastically reduces the formation temperature of the antecedent intermetallic phases, without forming excess binary carbides or facilitating powder oxidation in the Ti-Al-C and Zr-Al-C systems. This work elucidates the effect of metal hydrides on the sequence of formation reactions in MAX phase ceramics. In the Zr-Al-C system, the use of coarse, oxidation-prone elemental Zr powders prevented MAX phase formation, whereas spark plasma sintering of ZrH2 powders at 1500°C produced ceramics containing 60 wt% Zr3AlC2. Similarly, in the Ti-Al-C system, spark plasma sintering of TiH2 powders at 1200°C produced phase-pure Ti3AlC2 ceramics.

For the first time, MAX phases in the Hf-Al-C system were experimentally synthesised using reactive hot pressing. HfC was observed as the main competing phase. The lattice parameters of Hf2AlC and Hf3AlC2 were determined by Rietveld refinement based on the X-ray diffraction data. The atomic stacking sequence was revealed by high-resolution scanning transmission electron microscopy. Mixtures of 211 and 312 stacking were observed within the same grain, including 523 layers. This transition in atomic structure is discussed.

This study reports on the synthesis and characterization of MAX phases in the (Zr,Ti)n+1AlCn system. The MAX phases were synthesized by reactive hot pressing and pressureless sintering in the 1350-1700°C temperature range. The produced ceramics contained large fractions of 211 and 312 (n=1 and 2) MAX phases, while strong evidence of a 413 (n=3) stacking was found. Moreover, (Zr,Ti)C, ZrAl2, ZrAl3, and Zr2Al3 were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegard's law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegard's law around the (Zr0.33,Ti0.67)3Al1.2C1.6 stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed M6X octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase. The decomposition of the secondary (Zr,Ti)C phase was attributed to the miscibility gap in the ZrC-TiC system.

This study reports on the first experimental evidence of the existence of the Zr2AlC MAX phase, synthesised by means of reactive hot pressing of a ZrH2, Al and C powder mixture. The crystal structure of this compound was investigated by X-ray and neutron diffraction. The lattice parameters were determinedand confirmed by high-resolution transmission electron microscopy. The effect of varying the synthesis temperature was investigated, indicating a relatively narrow temperature window for the synthesis of Zr2AlC. ZrC was always present as a secondary phase by hot pressing in the 1475–1575?C range.

Herein we report, for the first time, on the synthesis and structural characterization of the Zr-based MAX phase, Zr3AlC2, fabricated by reactive hot pressing of ZrH2, Al, and C powders. The crystal structure of Zr3AlC2 was determined by X-ray diffraction and high resolution scanning transmission electron microscopy to be the hexagonal space group P63/mmc. The a and c lattice parameters of Zr3AlC2 are 3.33308(6) Å and 19.9507(3) Å, respectively. The samples include the secondary phases ZrC and Zr-Al intermetallics as confirmed by quantitative electron probe microanalysis. The Vickers hardness (3 kg) of Zr3AlC2 was measured to be 4.4 ±0.4 GPa.

The present work describes a synthesis route for bulk Ta4AlC3 MAX phase ceramics with high phase purity. Pressure-assisted densification was achieved by both hot pressing and spark plasma sintering of Ta2H, Al and C powder mixtures in the 1200–1650°C range. The phases present and microstructures were characterized as a function of the sintering temperature by X-ray diffraction and scanning electron microscopy. High-purity α-Ta4AlC3 was obtained by hot pressing at 1500°C for 30 min at 30 MPa. The β-Ta4AlC3 allotrope was observed in the samples produced by SPS. The Young’s modulus, Vickers hardness, flexural strength and single-edge V-notch beam fracture toughness of the high-purity bulk sample were determined. The thermal decomposition of Ta4AlC3 into TaCx and Al vapour in high (10−5 mbar) vacuum at 1200°C and 1250°C was also investigated, as a possible processing route to produce porous TaCx components.

New bulk MAX phase-based ceramics were synthesized in the Ta-Hf-AI-C and Ta-Nb-AI-C systems. Specifically, (Ta1-x,Hfx)4AlC3 and (Ta1-x,Nbx)4AlC3 stoichiometries with x = 0.05, 0.1, 0.15, 0.2, 0.25 were targeted by re­ active hot pressing of Ta2H, HfH2, NbH0.89, Al and C powder mixtures at 1550°C in vacuum. The produced ceramics were characterized in terms of phase composition and microstructure by X-ray diffraction, scanning electron microscopy, electron probe microanalysis and scanning transmission electron microscopy. The investigation confirmed the existence of such M-site solid solutions with low solute concentrations, as predicted by first-principles calculations. These calculations also predicted a linear trend in lattice parameter evolution with increasing Hf concentration, in agreement with the experimental results. ln order to increase the low phase purity of the produced ceramics, Sn was added to form (Ta1-x,Hfx)4(Al0.5,Sn0.5)C3 and (Ta1-x,Nbx)4(Al0.5,Sn0.5)C3 double solid solutions, thus resulting in a higher content of the 413 MAX phase compounds in the produced ceramics.

The addition of Nb and Sn to Zr2AlC is investigated, targeting the synthesis of a Zr-rich bulk MAX phase free of ZrC. The 211 phase formation in the two quaternary Zr-Nb-Al-C and Zr-Al-Sn-C systems is evaluated. Solubility over the entire compositional range in (Zr, Nb)2AlC and Zr2(Al, Sn)C is observed. In terms of effectiveness, the addition of Sn is preferred over the addition of Nb, as the former is selectively incorporated into the 211 structure. A combinatorial approach results in the formation of phase-pure (Zr0.8,Nb0.2)2(Al0.5,Sn0.5)C. The effect of the added solutes on the microstructure and crystallographic parameters is investigated. The addition of Nb and Sn reduces the distortion parameter of the trigonal prism compared to pure Zr2AlC. Therefore, an attempt is made to establish a more general stability criterion for the M2AC structure based on the steric relationship between the atoms in the M6A trigonal prism. Inspired by the Hume-Rothery rules, it is suggested that comparable atomic radii of the M- and A-atoms provide a good starting point to obtain a stable 211 MAX phase.

This work is a first assessment of the radiation tolerance of the nanolayered ternary carbides (MAX phases), Zr3AlC2, Nb4AlC3 and (Zr0.5,Ti0.5)3AlC2, using proton irradiation followed by post-irradiation examination based primarily on X-ray diffraction analysis. These specific MAX phase compounds are being evaluated as candidate coating materials for fuel cladding applications in advanced nuclear reactor systems. The aim of using a MAX phase coating is to protect the substrate fuel cladding material from corrosion damage during its exposure to the primary coolant. Proton irradiation was used in this study as a surrogate for neutron irradiation in order to introduce radiation damage into these ceramics at reactor- relevant temperatures. The post-irradiation examination of these materials revealed that the Zr-based 312-MAX phases, Zr3AlC2 and (Zr0.5,Ti0.5)3AlC2 have a superior ability for defect-recovery above 400°C, whilst the Nb4AlC3 does not demonstrate any appreciable defect recovery below 600°C. Density functional theory calculations have demonstrated that the structural differences between the 312 and 413- MAX phase structures govern the variation of the irradiation tolerance of these materials.

Guided by predictive theory, a new compound with chemical composition (Cr2/3Zr1/3)2AlC was synthesized by hot pressing of Cr, ZrH2, Al, and C mixtures at 1300°C. The crystal structure is monoclinic of space group C2/c and displays in-plane chemical order in the metal layers, a so-called i-MAX phase. Quantitative chemical composition analyses confirmed that the primary phase had a (Cr2/3Zr1/3)2AlC stoichiometry, with secondary Cr2AlC, AlZrC2, and ZrC phases and a small amount of Al−Cr intermetallics. A theoretical evaluation of the (Cr2/3Zr1/3)2AlC magnetic structure was performed, indicating an antiferromagnetic ground state. Also (Cr2/3Hf1/3)2AlC, of the same structure, was predicted to be stable.

Decomposition of Cr2AlC deposited onto a Zr substrate and vacuum-annealed is observed at 800°C as Al diffuses from the MAX phase into the Zr substrate. A double layer of ZrN and AlN has been predicted by CALPHAD calculations to act as diffusion barrier between the Zr substrate and Cr2AlC. Experimental thermal stability investigations corroborate this prediction by confirming that the proposed double layer diffusion barrier coatings suppress the decomposition of Cr2AlC for one hour at temperatures of up to 1000°C.

The synthesis of high-purity Mn+1AXn (MAX) phase ceramics in the Hf-Al-C ternary system from near-stoichiometric feedstock powder mixtures has been exceptionally challenging due to the rapid concurrent formation of persistent, ultrafine HfC impurities. This work synthesized ceramics containing the nanolaminated Hf5Al2C3 ‘superstructure’, showing that it comprises alternating Hf2AlC and Hf3AlC2 atomic stackings. For the first time, the Hf5Al2C3 complex structure was associated with the topotactic transformation of Hf2AlC into Hf3AlC2, which is observed upon heating the powder compact to temperatures higher than 1500°C; moreover, an inverse decomposition reaction of Hf3AlC2 into Hf2AlC was observed as result of further heating the powder compact to temperatures exceeding 1600°C. The crystal structure and lattice parameters of the Hf5Al2C3 ‘superstructure’ were determined. MAX phase ceramics containing up to 40–45 wt% Hf3AlC2/Hf2AlC were produced with HfC as the main competing phase. The hardness and damage tolerance of these MAX phase ceramics were also evaluated.

The deployment of Gen-IV lead-cooled fast reactors requires a good compatibility between the selected structural/cladding steels and the inherently corrosive heavy liquid metal coolant. An effective liquid metal corrosion mitigation strategy involves the in-situ steel passivation in contact with the oxygen-containing Pb-alloy coolant. Transmission electron microscopy was used in this work to study the multi-layered oxide scales forming on an austenitic stainless steel fuel cladding exposed to oxygen-containing (CO ≈ 10−6 mass%) static liquid lead-bismuth eutectic (LBE) for 1000 h between 400 and 500°C. The oxide scale constituents were analyzed, including the intertwined phases comprising the innermost biphasic layer.

Exposure of austenitic stainless steels to liquid lead-bismuth eutectic with low concentration of dissolved oxygen typically results in selective leaching of highly-soluble alloying elements and ferritization of the dissolution-affected zone. In this work, focused ion beam, transmission electron backscatter diffraction and scanning transmission electron microscopy were utilised to elucidate early-stage aspects of the dissolution corrosion process of cold-worked austenitic stainless steels exposed to static, oxygen-poor liquid lead-bismuth eutectic at 450C for 1000 hours. It was found that deformation-induced twin boundaries in the cold-worked steel bulk provide paths of accelerated penetration of the liquid metal into the steel bulk.

MXenes are electrically conductive 2D transition metal carbides/nitrides obtained by the etching of nanolaminated MAX phase compounds, followed by exfoliation to single- or few-layered nanosheets. The mainstream chemical etching processes have evolved from pure hydrofluoric acid (HF) etching into the innovative “minimally intensive layer delamination” (MILD) route. Despite their current popularity and remarkable application potential, the scalability of MILD-produced MXenes remains unproven, excluding MXenes from industrial applications. This work proposes a “next-generation MILD” (NGMILD) synthesis protocol for phase-pure, colloidally stable MXenes that withstand long periods of dry storage. NGMILD incorporates the synergistic effects of a secondary salt, a richer lithium (Li) environment, and iterative alcohol-based washing to achieve high-purity MXenes, while improving etching efficiency, intercalation, and shelf life. Moreover, NGMILD comprises a sulfuric acid (H2SO4) post-treatment for the selective removal of the Li3AlF6 impurity that commonly persists in MILD-produced MXenes. This work demonstrates the upscaled NGMILD synthesis of (50 g) phase-pure Ti3C2Tz MXene clays with high extraction yields (>22%) of supernatant dispersions. Finally, NGMILD-produced MXene clays dry-stored for six months under ambient conditions experience minimal degradation, while retaining excellent redispersibility. Overall, the NGMILD protocol is a leap forward toward the industrial production of MXenes and their subsequent market deployment.